1. Field of the Invention
The present invention relates to a direct dialing-in method in a private automatic branch exchange (PABX) which has the user-network interface layer of the integrated services digital network (ISDN), especially to the direct dialing-in method which is capable of discriminating the attributes of a variety of terminals.
2. Description of the Prior Art
In a conventional PABX connected to ISDN, an extension number is determined by a called party number information element included in a SETUP message in accordance with a called party number translation table 600 as shown in FIG. 6.
The called party number translation table 600 stores the number of digits 601 in a direct dialing-in number from which is derived the extension number in a PABX at a called side, a first and a last number (602f and 6021) for defining the extent of a called party number group with the above number of digits, and the first extension number corresponding to the first called party number.
When a calling party dials the dialing-in number of a called party, this direct dialing-in number is set in the called party number information element of the SETUP message and is sent out from a PABX at a calling side to a PABX at a called side across ISDN. When the SETUP message is received on the PABX in a called side, a central processing unit 31 in a PABX extracts a called party number having the number of digits which is specified by the column 601 of a called party number translation table 600 from the above direct dialing-in number. Then, the central processing unit 31 determine, to which address group the called party number belongs in the column 602, 604, . . . on the above called party number translation table 600.
From the difference between the called party number and the first called party number of the address group including the called party number, a relative address in the address group is calculated. The relative address is added to the first extension number. As a result, the sum thus obtained is an extension number of the called terminal. Now, an actual example will be explained of the conventional direct dialing-in method in the case mentioned below:
The direct dialing-in number on the contract with ISDN: 930-1231.
An extension number of a telephone: 231.
______________________________________ Content of called party number translation table ______________________________________ the number of digits of called party number 4 the first called party number 1200 the last called party number 1300 the first extension number for called terminal 200 ______________________________________
In this case, the direct dialing-in number received by the PABX is 9301231. The four least significant digits (=1231) of the direct dialing-in number become a called party number. This called party number is included in an address group assigned by the first called party number (=1200) and the last called party number (=1300). Accordingly, the difference between the called party number (=1231) and the first called party number (=1200) is 31. This difference (=31) is added to the first extension number (=200), and the result of addition (=231) is the extension number of the called terminal.
In the above conventional direct dialing-in method, however, the called party number translation table 600 is prepared in such a manner that the direct dialing-in number is disposed on a one-to-one basis for the extension number of the PABX. Accordingly, in a system where a PABX accommodates a variety of terminals such as a telephone and a data terminal etc., it arises the problem that different direct dialing-in numbers are required for a plurality of terminals of a PABX.